1. Field of the Invention
The invention relates to a method and apparatus for reducing the power dissipation, in the form of switching losses, in a switching device. More specifically, the invention relates to a method and apparatus for estimating the temperature of the switching device controlling an electric device, such as a motor, in order to operate the switching device at an optimal switching frequency (i.e. controlling or sliding the frequency) to reduce switching losses for purposes of thermal protection.
2. Description of the Related Art
It is desirable to provide thermal protection for a semiconductor switching device by maintaining it at or below a certain maximum temperature limit both during normal operation and during the switching operation, when the switching device is switched ON or OFF. The thermal protection of a switching device, which may have high power dissipation under both stable and transitory operating conditions, better ensures temperature stability and, therefore, reliable operation. Conventional methods of providing thermal protection for a switching device involves measuring the actual temperature of the switching device, and adjusting the switching frequency to an optimal level based on the measured temperature.
For such purposes, "switching frequency" refers to the rate or interval at which the switching device, such as a transistor, is switched ON or OFF. For example, if a transistor is switched ON and OFF continuously at a 1 millisecond interval, it is said to have a 1 kHz switching rate. During each switching interval, power is generated by the switching device as a result of slew rate of the voltage and the current supplied to the switching device. This generated power is dissipated in the form of losses in the switching device. The total power losses of a switching device consist of switching loss and conduction loss. The magnitude of the switching loss is a function of the switching frequency and the amount of the current (I) conducted through the switching device. The magnitude of the conduction loss is a function of the amount of current (I) passing through the switching device. Power generated by such losses is absorbed by the switching device in the form of thermal energy, or heat, which typically increases the temperature of the switching device, and in turn must be dissipated from the switching device to the environment by heat transfer methods, such as radiation, convection and conduction.
FIG. 1 shows typical voltage and current curves for a typical semiconductor switching device. As shown in FIG. 1, when the switching device is turned ON, the voltage V drops at a particular slew rate (or slope) between times T.sub.1 (the time the switching device is provided the ON signal) and T.sub.2 (the time the switching device is in the steady-state of ON), and the current I increases at a particular slew rate during this same time period. The interval between times T.sub.1 and T.sub.2 corresponds to the switching ON time. When the switching device is turned OFF, the voltage increases at a particular slew rate, and the current decreases at a particular slew rate between times T.sub.3 (the time the switching device is provided the OFF signal) and T.sub.4 (the time the switching device is in the steady-state of OFF). The interval between times T.sub.3 and T.sub.4 corresponds to the switching OFF time.
In FIG. 1, the slew rates are shown as being the same for the ON and OFF times for both the current and voltage levels in the switching device. However, this need not necessarily be the case, and all slew rates may vary based upon whether the switching device is switched ON or OFF. For example, the time it takes the voltage to drop from its high (OFF) level to its steady-state low level (ON) between times T.sub.1 and T.sub.2 may be less than the time it takes the voltage to increase from the steady-state low level to the high level between times T.sub.3 and T.sub.4.
When the switching device is operating in its normal state (i.e., during the steady-state period between an ON or an OFF switching time interval), there is a current value of I.sub.L and a voltage value of V.sub.L being supplied to the switching device, and this is shown between times T.sub.2 and T.sub.3 of FIG. 1. The switching loss corresponds to the power generated during interval from times T.sub.1 to T.sub.2 (when the switching device is turning ON) and during the interval from times T.sub.3 to T.sub.4 (when the switching device is turning OFF), while the conduction loss corresponds to the power generated during the interval between times T.sub.2 and T.sub.3 (when the switching device is ON).
The time or interval between consecutive turning ON and turning OFF of the switching device defines the switching rate, or switching period. At a faster switching rate, the switching device has less time between switching intervals to dissipate through heat transfer to the environment, the total power generated by the switching operation and absorbed as thermal energy within the switching device during each switching period. As a result, the switching device will be absorbing more thermal energy than it is transferring to the environment and its temperature typically will increase as a result of the increasing residual between power generated and thermal energy dissipated. (Note that the heat generated during the switching periods is typically much greater than the heat generated during the conduction periods; however, since the switching times (i.e., the interval between times T.sub.1 and T.sub.2 and times T.sub.3 and T.sub.4) are typically smaller than the normal operating periods of the switching device, this does not present too much of a problem.) As a result, when the switching device is operating at a high switching rate, the temperature of the switching device may increase beyond acceptable limits.
U.S. Pat. No. 4,727,450, entitled "Temperature Measuring, Protection and Safety Device, Thermal Protection Device Using the Temperature Measuring Device and Electronic Power Controller Using the Thermal Protection Device" (the '450 patent), issued on Feb. 23, 1988, describes a method for controlling the temperature of an electronic power controller.
The device disclosed in the '450 patent utilizes an initialization device for setting a thermal model to an operating point representative of the actual thermal state of the circuit to be protected. The real (actual) temperature of the electronic power controller is measured and, based on that temperature, appropriate thermal protection is effected by cutting off power to the circuit. The device disclosed in the '450 patent also utilizes a thermometer to set the initial conditions in the initialization circuit.
Although the device of the '450 patent may provide thermal protection for a switching device, it is desirable to have a thermal protection device that does not cut off power to the circuit when the temperature of the switching devices gets too high, but instead adjusts the switching frequency of the switching device in a manner that allows continued operation without risk of thermal damage to the switching device.
Further, in many applications it is undesirable to have a thermal protection device which requires an initialization device in order to set up the thermal model for determining if the device is operating at too high a temperature, as is disclosed in the '450 patent.